Having personal knowledge of Mike's tube business and his associated
test equipment and inventory (Mike and I were college classmates, and
I've known him for more than twenty years), I feel that I can safely
say that what he claims is valid.
My Sylvania Technical Manual (1982 edition) confirms that their 6BG6GA
is electrically identical to their 6L6GC/7581 types. Mike has confirmed
this through exhaustive tests on, among other things, his vacuum tube
bridge.
Insofar as the claim to be the same as the 7027A is concerned, the only
way to determine that is to look a the tubes' screen grid mechanical
design / structure, which I have. They are the same. QED.
Mike never claims that these Sylvania 6BG6GAs are plug-in replacements
for 6L6GC / 7581 / 7027A types, of course you either have to add an
anode lead/cap to your equipment or use one of the adapter kits he
sells. And of course neither of these would work if your equipment
can't accommodate the increase in height due to these parts (seldom a
problem in audio/music amplifiers, I might add). Mike states all of
this on his Web site.
The use of identical tube elements in different tube envelopes / bases
was of course common practice In Those Days; as another example, the
6JB6 and 6GJ5 are electrically identical but were manufactured with a
slightly different base (at least in Sylvania-branded tubes).
I'm sorry if I seem like I'm a bit testy. Mike takes his tube business
and his customer reputation very seriously (he works HARD at it and puts
up with a lot of crap from audiophools, etc. just to sell tubes) and is
also well-respected by his competitors, so after reading some of today's
digest posts I felt the need to speak up for him a bit.
Peace.
Mahlon - K4OQ